Ribonucleotide reductase will be further studied as an example of a class of non-classical regulatable enzymes which are monomeric, but demonstrate feedback inhibition or other regulatory properties. The problem to be investigated are the structural relationship of the regulatory site to the catalytic site, the nature of the conformational changes that occur when ligands bind to the regulatory site, and in particular the mechanism by which binding of different ligands at the regulatory site alters the velocity of reaction of different specific substrates at the catalytic site. As a corollary of this investigation the mechanism of the reaction at the catalytic site will be further explored.